Scalar Potential

Consider a cube of edge a. There is a point charge q at each corner. Find [tex]\phi[/tex] at the center of the face for which x=a.
The answer to the problem is [tex]\0.707*(q/\epsilon_0*a)[/tex]
I have to use the the scalar potential equation, but I have been stuck on this problem for a while. I know that I have to consider the angle which is cos45 = 0.707. I am not sure I understand what they are asking in the problem.